1. Field of the Invention
The invention relates to a power converter, more particularly to a soft-start switching power converter.
2. Description of the Related Art
Referring to FIG. 1, a conventional switching power converter 1 which receives an alternating-current (AC) voltage from an AC power source 10, and which outputs a direct-current (DC) output voltage based on the AC voltage is illustrated. The conventional switching power converter 1 includes a rectifier circuit 11, a voltage converter circuit 12 and a control signal generator module 13.
The rectifier circuit 11 is connected electrically to the AC power source 10 for receiving the AC voltage from the AC power source 10, and rectifies the AC voltage so as to generate a rectified voltage VRE.
The voltage converter circuit 12 is connected electrically to the rectifier circuit 11 for receiving the rectified voltage VRE, receives a control signal, adjusts the rectified voltage VRE based on the control signal so as to output the DC output voltage, and generates an output signal, a divided voltage signal and a driving voltage. The voltage converter circuit 12 includes a voltage converter 120, a series connection of two resistors 121 and 122, a capacitor 123, a switch 124, an output unit 125 and a voltage divider unit 126.
The voltage converter 120 is connected electrically to the rectifier circuit 11 for receiving the rectified voltage VRE. The resistors 121 and 122 are connected between the rectifier circuit 11 and the capacitor 123, and the capacitor 123 is grounded. A portion of a current flowing through the resistors 121 and 122 charges the capacitor 123. A voltage across the capacitor 123 serves as the driving voltage. A series connection of the switch 124 and the output unit 125 is grounded, and is connected electrically to the voltage converter 120. The switch 124 is controlled by the control signal to switch between conduction and nonconduction. The output unit 125 outputs generates the output signal based on a current flowing through the switch 124. The voltage divider unit 126 is grounded and is connected electrically to the voltage converter 120 for generating the divided voltage signal.
The control signal generator module 13 is connected electrically to the voltage converter circuit 12 for receiving the divided voltage signal, the output signal and the driving voltage. When the driving voltage reaches a predetermined start-up voltage, the control signal generator module 13 generates the control signal based on the divided voltage signal, the output signal and the driving voltage, and outputs the control signal to the switch 124.
The conventional switching power converter 1 has the following disadvantages:
1. Longer time is required for start-up. Since resistance of the resistors 121 and 122 is relatively high, the current flowing through the resistors 121 and 122 is relatively small, such that when the capacitor 123 is charged by the current flowing through the resistors 121 and 122, about one to three seconds are required to enable a voltage across the capacitor 123 (i.e., the driving voltage) to reach the predetermined start-up voltage. Therefore, longer time is required for the switching power converter 1 to start up.
2. More power is required by the conventional switching power converter 1. Since an overall resistance of the resistors 121 and 122 is around 0.3 M ohms, power P consumed by the resistors 121 and 122 is equivalent to a product of a voltage V across the resistors 121 and 122, and the current I flowing through the resistors 121 and 122 (i.e., P=I×V), the conventional switching power converter 1 consumes more power owing to the resistors 121 and 122.